X-ray or gamma ray use in control of crystal diameter



March 10, 19-70 (a. ZWANEBURG 3,499,736

} X-RAY 0R GAHMA RAY USE IN CONTROL OF CRYSTAL DIAMETER Filed Oct. 5. 1966 V FIG. I, I L

- 1 HIGH z FREQuENcY 7 GENERATOR )30 we? n 31 E f IX-RAY IMAGE *%8%Y=%.JEB 5- "*:%MAGE f X-RAY SCREEN 1 y m k\\ 2 s FIG.2 f k United States Patent 3,499,736 X-RAY OR GAMMA RAY USE IN CONTROL OF CRYSTAL DIAMETER Gooitzen Zwanenburg, Emmasingel, Eindhoven, Netherlands, assignor, by mesne assignments, to US. Philips Corporation, New York, N.Y., a corporation of Delaware Filed Oct. 5, 1966, Ser. No. 584,584 Claims priority, application Netherlands, Oct. 6, 1965, 6512921 Int. Cl. Btilj 17/20 U.S. 'Cl. 23301 4 Claims The invention relates to a method for pulling crystals from a melt in a crucible, in which the diameter of the growing crystal is controlled and to a device suitable for carrying out said method.

The manufacture of crystals by pulling from a melt is proposed by Czochralski; it is used inter alia for obtaining single crystals consisting of fusible semi-conductor material, for example germanium, silicon and fusible compounds of the type A B for example, gallium arsenide. There is a particular interest in rod-shaped crystals of a constant diameter over a large part of their length. Semiconductor crystals of such a shape are used inter alia for the manufacture of semiconductor devices such as transistors and diodes. For this purpose, such a crystal which usually has a diameter of the order of l to 3 cms. and a length of about one to a few decimetres is divided into waferor strip-shaped bodies, for example by sawing, on which bodies one or more semiconductor devices are formed.

In the manufacture of such single crystals it is important that the diameter of the growing portion should be controlled in the desired manner with optimum accuracy, particularly that the diameter of the crystal should have a constant value over large part of its length. It is known that this diameter depends upon various conditions chosen for the growth, for example, the temperature of the melt and the pulling rate and it has been proposed to adjust the desired diameter or the desired varia tion of this diameter by correct adjustment of said conditions.

Thecontrol of the diameter involves, however, the difficulty that the growth region of the crystal is located inside the crucible and that the diameter of the growing crystal portion and the possible variation thereof can be checked only by observation via the open upper side of the crucible. This observation is the more difiicult the lower becomes the level of the melt in the crucible and the larger is the diameter of the crystal.

The present invention has for its object inter alia to obviate the aforesaid disadvantages.

The method of pulling crystals from a melt in a crucible, in which the diameter of the growing crystal is controlled, is characterized in accordance with the invention in that the diameter of the growing crystal is determined and/or controlled by means of X-rays or gamma rays passing through the wall of the crucible. In a preferred embodiment an image comprising the shadow of the growing crystal portion, projected outside the crucible by the X-rays or gamma rays may be converted into an image of visible light, for example, by means of a fluorescent screen sensitive to the radiation concerned, if necessary with image intensification or by using a pick-up tube sensitive to the radiation concerned which converts the radiation image into an electrical signal which is used for forming an image of visible light in a display tube. It is thus possible by means of the optical reproduction of the growing crystal portion to control the course of growing and, if desired, to interfere. The factor of magnification of the image may be readily determined in advance, for ex- 'ice ample, by means of the dimensions of the apparatus or by calibration with one or more rods of known diameters. The image of the growing portion is not affected by the crystal portions lying above said portion or by the crystal holder.

Since it is thus possible to form an image of the growing portion outside the apparatus, the surface of said portion forming a boundary between an irradiated part and a shaded region, use may be made of one or more optoelectrical converters for keeping the diameter constant, the signals provided by said converters being employed for adjusting, for example, the temperature of the melt or the pulling rate. The term opto-electrical converters" is not restricted to converters responding to visible light, but it includes also converters capable of converting X-rays or gamma radiation into electrical signals, for example, Geiger counters or scintillation counters.

Converters of the last-mentioned kind are capable of responding directly to a displacement of the shade boundary of the growing rod portion, whilst other opto-electrical converters, which respond for example to visible light, may be employed in conjunction with an image converter which converts the image projected by the X-rays or gamma rays into an image of visible light. Image intensification and, if desired, optical magnification, for example, by means of lenses may be carried out.

By means of the aforesaid converters and of controlcircuits responding to the signals from said converters one or more of the parameters of the pulling apparatus determining the diameter of the growing crystal particularly the increase or the decrease of said diameter, for example the degree of heating of the melt and the pulling rate can be acted upon, in order to reset the diameter in the case of a small deviation therefrom.

The invention furthermore relates to a device for pulling crystals from a melt, said device comprising a crucible for the melt, a holder vertically movable with respect to the crucible for pulling a crystal, heating means for the melt and control-means for adjusting and/ or varying one or more of the parameters acting upon the diameter of the crystallizing portion, for example the degree of heating of the melt and the pulling rate. This device is provided, in accordance with the invention, with a source of X-rays or gamma rays, disposed at the side of the crucible and a receiver arranged on the opposite side of the crucible which converts the radiation into a visible image and/0r into one or more electrical signals.

The present invention permits inter alia of manufacturing single crystal rods of semiconductor material, for example, germanium or silicon, 0f diameters of more than 25 mms. for example 30 to 50 mms., the rod diameter being kept accurately constant with a tolerance of 1 mm. Since the inner diameter of the crucible need no longer be chosen so large as would be desirable for a satisfactory observation from above, the horizontal dimensions of the crucible and of the heating means, for example, a high-frequency coil, with respect to the known apparatus for maximum crystal diameters of 15 to 25 mms. need not be enlarged proportionally to the relevant crystal diameters. The hindrance of observation is no longer a factor determinative of the height of the crucible, so that the invention furthermore permits of obtaining longer crystals, or particularly when forming crystals of large diameters crystals of reasonable length.

it is furthermore possible to maintain the desired crystal diameter down a small remainder of the melt only. It is then possible to cause the remainder to grow completely to the lower end of the crystal by gradually reducing the diameter, and as the case may be further with the reduced diameter, so that it is avoided that the remainder solidifies afterwards in the crucible, which might cause rupture of the crucible. Such a gradual reduction of 3 the diameter at theend of the crystal could not well be checked by observation according to known art.

The invention will be described more fully with reference to the accompanying drawing, in which FIG. 1 shows diagrammatically a vertical sectional view of an embodiment of a device for pulling crystals and FIG. 2 is a front view of the X-ray image screen designed in FIG. 1 in the direction of the arrows.

The apparatus shown in FIG. 1 comprises a quartz tube 1, having an outer diameter of 130 mms. and a wall thickness of 2.5 mms. The lower end and the upper end of the tube are provided with closing members 2 and 3, respectively, in which gas inlet and gas outlet channels (not shown) are provided. On the upper side and on the lower side two shafts 4 and 5 respectively are taken through the wall in a gas-tight manner so as to be vertically movable and rotatable about their centre lines. The lower shaft 5 ness. The crucible 9 is surrounded by a heat-insulating sheath 14 of graphite felt of 5 mms. thickness. The upper shaft 4 is provided at the lower end with a crystal holder 11 for the crystal to be drawn.

At the level of the crucible the tube 1 is surrounded by a. cylindrical high-frequency coil 12 formed by a single winding and a cylindrically curved, copper plate. The coils has an inner diameter of 135 mms. and a height of 130 mms. and is provided with a welded copper cooling duct 13. The high-frequency coil is connected to a high-frequency generator 32.

Outside the high-frequency coil 12 an X-ray tube and a diaphragm 21 are provided. The X-ray tube may be a commercially available tube of the trademark Practix 90/20 type XB 2000, which can be driven continuously at a voltage of 70 kv. and a current intensity of 0.8 ma;

On the side of the high-frequency coil 12 opposite the X-ray tube 20 an X-ray screen 22 is arranged, which is sensitive to X-rays and which produces a visible image picture of the image projected by the X-rays onto the screen.

The X-ray tube 20 and the diaphragm 21 are disposed so that the X-beam emanating from the centre of the diaphragm is horizontally directed to the axis of the cylindrical crucible. The cylindrical high-frequency coil 12, having a wall thickness of 2 mms., comprises two copper windows 23 and 24 of a thickness of 0.15 mm. for passing the X-ray beam towards the image screen.

The apparatus comprises furthermore a control for manual adjustment and control of the driving device 7 for the vertical movement of the shaft 4 with the crystal holder 11, a control 31 for the manual adjustment and control of the high-frequency generator 32 supplying the current to the'high-frequency coil 12 and a control 33 for the manual adjustment and control of the driving device 6 for the vertical movement of the shaft 5 with the support 8 and the crucible 9.

This apparatus may be used as follows for growing a crystal, for example, of germanium or silicon.

A quantity of a material for a crystal to be made, for example germanium or silicon is put in the crucible 9 and a seed crystal 40 is fastened to the crystal holder 11. The high-frequency coil 12 is energized so that the crucible 9 is heated and the crystal material in the crucible is melted. The crucible is simultaneously rotated about its vertical axis. Then the X-ray tube is ignited and driven continuously with a current intensity of 0.8 mat. and a direct voltage of 70 kv.

The shaft 5 is displaced vertically so that the meniscus of the melt 41 in the crucible is located approximately at the level of the anode of the X-ray tube 20 and the diaphragm 21, so that an image of this meniscus appears on the screen 22 and the correct height can be adjusted. The shaft 4 with the crystal holder 11 and the seed crystal 40 is then moved downwards to an extent such that the lower end of the seed crystal is just above the melt, which can be observed from the shadow image on the screen 22. The seed crystal 40 is preheated by the radiation from the melt and the wall of the crucible. Then the shaft 4 is moved further downwards until the lower end of the seed crystal is dipped into the melt. The seed crystal is then gradually pulled upwards, whilst the rate of the upward movement of the shaft 4, and by controlling the supply current of the high-frequency generator 32, the temperature of the melt are adjusted so that the diameter of the portion growing on the seed crystal 40 is gradually reduced, after which it is kept constant whilst the image screen 22 is observed. In the thin portion 42 of the growing crystal thus formed any dislocations of the seed crystal grow for the major part towards the surface of the thin crystal portion. The temperature of the melt is subsequently reduced, so that the crystal diameter widens conically to the desired rod diameter.

When the desired diameter is attained, which can be determined by means of the image of the growing portion of the crystal on the screen 22, the pulling rate of the crystal and the temperature of the melt are adjusted so that a further growth with a substantially constant diameter is obtained. An accurate check of the diameter of the portion 43 of the growing crystal is rendered possible by the picture on the screen 22.

FIG. 2 shows clearly the screen 22 with the picture of the lower portion 53 of the growing crystal and the portion of the melt 51 from which the crystal is drawn. The diameter of the growing portion can be accurately determined by means of a calibration 55 on the screen. Since during the growth of the crystal the meniscus of the melt in the crucible falls down at a rate depending upon the quantity of crystallized material per unit time, the crucible is moved upwards by means of the shaft 5 and the driving mechanism 6 so that the height of the meniscus 56 reproduced on the screen 22 is kept constant.

In the event of an increase or a reduction of the crystal diameter said diameter can be returned to the desired value by means of the controls 30 and/or 31.

When the meniscus of the melt has arrived at the proximity of the bottom of the crucible, the rod diameter can be gradually reduced by raising the pulling rate and/ or by raising the temperature of the melt, so that a conical portion grows on, the reduced diameter being then maintained until the portion 43 of constant diameter just emerges from the upper edge of the crucible. The portion 43 is thus cooled in the same manner throughout its length, so that the crystal prefection is as uniform as possible throughout its length. Finally, the last molten rest of the material to be crystallized is grown on the crystal end.

By means of the observation of the screen 22 the aforesaid growth of the last parts below the portion 43 of constant diameter can be observed so that the contact between the crystal and the melt is not interrupted. Such an observation is quite impossible by direct view from above the crucible.

It should be noted that, if a rest of the melt, for example, of silicon, is left in the crucible, the crucible or at least the inner quartz portion 10 thereof may crack, when the crucible cools down and the rest solidifies.

It is furthermore possible to carry out an automatic check after the small portion '42 has been drawn and the gradual increase of the diameter has been adjusted, or when this diameter is attained, for example by means of photocells behind the screen at the level of the edge of the growing portion, when the correct diameter is reached, said photocells controlling automatically the pulling rate and/or the current supply to the high-frequency coil by an appropriate control circuit so that deviations from the desired diameter are automatically corrected. Instead of using photocells sensitive to visible light of the screen, X-ray sensitive opto-electric converters may be employed, for example, in front of the screen, or when the screen has been temporarily removed. Such converters may be arranged in known manner and the control-circuits employed thereto are known as well. Then the growth of the rest of the melt may, if desired, be controlled by hand Whilst the picture on the screen 22 is observed. It is thus possible to obtain continuous operation of various growing apparatus, which can be controlled by one operator.

It is obvious that the invention is not restricted to the apparatus described here by way of example, many variants being possible within the scope of the present invention. It is furthermore evident, that the design of the apparatus should be adapted to the use of X-ray and gamma ray radiation, for example, by choosing the material and the thickness of the transmitting parts also with a view to the radiation used, for example the hardness thereof.

What is claimed is:

1. A method of pulling crystals from a melt in a crucible, in which the diameter of the growing crystal is controlled, characterized in that the diameter of the growing portion of the crystal is controlled responsive to X-rays or gamma radiation passing through the wall of the crucible and the grown crystal containing information of the diametral extent of the grown crystal.

2. A method as claimed in claim 1, characterized in that an image projected outside the crucible by the X-rays or gamma rays, including the shadow of the growing portion of the crystal, is converted into an image of visible light.

3. A method as claimed in claim 1, characterized in that the radiation image is converted into electrical signals by opto-electrical converters which respond to a displacement of the shadow boundary of the growing portion of the crystal for controlling one or more parameters acting upon the diameter of the crystal during growth.

4. An apparatus for pulling crystals from a melt in which the diameter of the growing crystal is controlled, said apparatus comprising a crucible for the melt, a holder for the crystal to be grown, said holder being vertically movable with respect to the crucible, heating means for the melt, and control-means for adjusting and varying at least one of the parameters acting upon the diameter of the crystallizing portion, said parameters including the degree of heating of the melt and the pulling rate, characterized in that said apparatus further comprises a source of X-rays or gamma radiation arranged at the side of the crucible and means arranged on the opposite side of the crucible for converting said radiation passing through the Walls of said crucible and projecting an image of the diametrical extent of the grown crystal into a visible image or electrical signal.

References Cited UNITED STATES PATENTS 2,992,311 7/ 1961 Keller.

3,001,070 9/1961 Davis et a1.

3,011,055 11/1961 Friedman 25052 3,046,379 7/ 1962 Keller et a1.

3,190,727 6/ 1965 Vunderink.

3,265,470 8/1966 Keller.

3,372,276 3/1968 Reynolds et a1. 25052 XR NORMAN YUDKOFF, Primary Examiner V. W. PRETKA, Assistant Examiner US. Cl. X.R. 23-273; 25059 32 27 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 349973 Dated March 10. 1970 Inv ntor(s) GOOI'IZEN ZWANENBURG It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 1, line 36, after "over" should be inserted -a---.

Column 1, line 61, the comma after "portion" should be cancelled Column 1, line 65, after "intensification" should be inserted a comma.

Column 2, line 66, after "down" should be inserted to--.

Column 3, line 9, "arrows" should read --arrow II--.

Column 3, line 39, after "tube" should be inserted -20--.

Column line 56, "prefection" should read --perfection--.

Signed and sealed this 8th day of p 1970 $EAL) Anew EdwardMFletchenIn WILLIAM E- SGHUYLER, JR.

Attesting Officer issioner of Patents 

1. A METHOD OF PULLING CRYSTALS FROM A MELT IN A CRUCIBLE, IN WHICH THE DIAMETER OF THE GROWING CRYSTAL IS CONTROLLED, CHARACTERIZED IN THAT THE DIAMETER OF THE GROWING PORTION OF THE CRYSTAL IS CONTROLLED RESPONSIVE TO X-RAYS OR GAMMA RADIATION PASSING THROUGH THE WALL OF 